[Beowulf] Mellanox ConnectX-3 MT27500 problems

Jörg Saßmannshausen j.sassmannshausen at ucl.ac.uk
Sat Apr 27 13:05:06 PDT 2013

Dear all,

I was wondering whether somebody has/had similar problems as I have.

We have recenctly purchased a bunch of new nodes. These are Sandybridge ones 
with Mellanox ConnectX-3 MT27500 InfiniBand connectors and this is where I got 
problems with.

I am usually using Debian Squeeze for my clusters (kernel 2.6.32-5-amd64). 
Unfortunately, as it turned out I cannot use that kernel as my Intel NIC is 
not supported here. So I upgraded to 3.2.0-0.bpo.2-amd64 (backport kernel to 
sqeeze). Here I got network but the InfiniBand is not working. The device is 
not even recognized by ibstatus. Thus, I decided to do an upgrade (not dist-
upgrade) to wheezy to get the newer OFED stack.

Here I get the InfiniBand working but only with 8.5 Gb/sec. A simple reseating 
of the plug increases that to 20 Gb/sec (4X DDR), which is still slower than 
the speed of the older nodes (40 Gb/sec (4X QDR)).

So I upgraded completely to wheezy (dist-upgrade now) but the problem does not 
I re-installed squeeze again and installed a vanilla kernel (3.8.8) and the 
latest OFED stack from their site. And guess what: same experiences here: 
After a reboot the IfniniBand speed is 8.5 and reseating the plug increases 
that to 20 Gb/sec. It does not matter whether I connect to the edge switch or 
to the main switch, in both cases I got the same experiences/observations.

Frankly, I am out of ideas now. I don't think the observed speed change after 
reseating the plug should happen. I am in touch with the technical support 
here as well but I think we both are a bit confused.

Now, am I right to assume that the Mellanox ConnectX-3 MT27500 are QDR cards 
so I should get 40 Gb/sec and not 20 Gb/sec?

Has anybody made similar experiences? Any ideas?

All the best from London


Jörg Saßmannshausen
University College London
Department of Chemistry
Gordon Street

email: j.sassmannshausen at ucl.ac.uk
web: http://sassy.formativ.net

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